甘肃陇西地区饱和黄土状土工程地质特性

Engineering geological characteristics of saturated Loessial Soil at Longxi area,Gansu

为了解甘肃陇西地区饱和黄土状土在细观结构下的强度降低原因并提出工程施工方案,通过现场实验测试和室内试验研究工程特性,利用显微镜观察了黄土状土的微观结构。结果表明：该地区黄土状土主要由上更新统风积（eol Q2^3）、洪积（plQ1^3）黄土状土、全新统洪积（pl Q1^4）黄土状土pl Q13,al-pl Q13的粉质土,粉质粘土,夹有轻粉质穿插透镜体构造组成,粉土（60%左右）为主,平均厚度为25 m的黄土状土层,底部下伏基岩。在地下水的作用下,形成饱和黄土状土体,具有小桥状连接、焊接状连接、嵌埋状连接的颗粒间连接方式,孔隙以粒间孔隙（包括支架孔隙和镶嵌孔隙）为主。饱和软塑状黄土状土的抗剪强度与硬塑状土相比有大幅度的降低,其凝聚力一般降低30%~50%,内摩擦角降低50%。在该饱和黄土状土分布区施工,建议先开挖导洞排水并及时支护,宜采用新奥法进行施工。

The purpose of this study is to understand the causes of the decreased strength of saturated loess in Longxi area of Gansu,and to put forward the construction scheme. Through field experiment and laboratory experiment study of Loess soil engineering geological characteristics,the microstructure of loess soil was observed with microscope. According to the result of field experiments and test analysis,loessial soils of this area is composed of aeolian(eol Q23) and alluvial(pl Q13) loessial soils at upper Pleistocene,and Alluvial(pl Q14) loessial soils(pl Q13) at Holocene. The al-pl Q13 is silt-clay soil,with light silt lenticle that made up with slit(about 60%). The average thickness of loess layer is 25 m,and bedrock is at the bottom. Under the action of underground water,loessial soil is saturated,and soil inter particle connection modes are bridge connection,welded connection and embedded connection. The pore is mainly interparticle porosity(including pores of the scaffolds and embedded voids). Saturated soft plastic loess soil's shear strength is much lower than hard plastic soil. Its cohesive force can reduce by 30% to 50%,and internal friction angle can reduce by 50%.It is recommended to excavate the guide hole and set up supporting before construction in this saturated loess distribution area. The New Austrian Tunneling Method(NATM) should be applied.